At first there were only a few nests. But over the last two decades, hundreds of leatherback sea turtles have made the beaches near the El Yunque national rain forest in northeastern Puerto Rico a popular Caribbean destination. Kike Calvo for The New York Times A baby leatherback turtle. Now, dozens of environmental groups are asking federal officials to designate the prime tourist development area a critical habitat for the endangered leatherback before other visitors threaten them. In a letter to the Interior secretary, Ken Salazar, and the Commerce secretary, Gary Locke, the groups, led by the Sierra Club, say that the “northeast ecological corridor” has become Puerto Rico’s largest nesting site for the leatherback, hosting more than 450 nests a year on three beaches. The groups say the population is now at risk because of a decision last October by Governor Luis Fortuñ...